Recent development and adaptation of optical techniques for application to communications, data processing, and similar signal transmission systems have numerous advantages as compared with electronic systems having analagous objectives.
Low-loss high quality multi-mode fiber optic cables have been developed which renders optical communication, data processing, and signal transmission systems feasible, practical, and desirable.
Some of the advantages of fiber optic cables used in such systems include greatly reduced size, weight, and cost, reduced constraints related to impedance matching, and easy coupling to common logic circuitry by the use of available light sources and light detectors. Moreover, fiber optic cables permit high channel-to-channel isolation, easy interchangeability with electrical cables, and greatly reduced electromagnetic interference.
One of the principal advantages of fiber optic cables in military use is their virtual immunity to radio frequency signals and their characteristic containment of transmitted signals wholly within the cables. In ground communication systems this greatly increases transmission security. Furthermore, the use of optical cables also eliminates cross-talk and the optical paths provided by such fiber optic cables are resistant to electromagnetic interferences from other equipment on an aircraft, vessel, or vehicle, even to the extent of being immune to electromagnetic pulses such as may be generated by nuclear explosions.
In optical systems there is a need for optical couplers which, when connected to fiber optic cables, for example, are capable of intercepting and extracting signal information from an optical path as well as introducing additional signal information into the optical path. It is also highly desirable, that such a coupler be adaptable to perform in a frequency selective manner so as to extract from or introduce into the optical path only light energy signals of certain predeterminable frequencies.
Since the terminal ends of fiber optic cable are known to emit transmitted energy in a conical pattern, it is desirable that such a coupler be designed to efficiently collect the conically emitted pattern of light energy which occurs when a fiber optic cable is parted for the insertion of a coupler for transferring light energy signals in and out of the fiber optic cable.